This invention relates to a semiconductor memory device, more particularly to a semiconductor memory device in which a volatile memory means and a non-volatile memory means are combined.
1. Field of the invention
Generally, a memory device which can retain data in the absence of power is called "a non-volatile memory device", and that which cannot retain data in the event of power removal is called "a volatile memory device". Both of these volatile and non-volatile memory devices can be constructed as a semiconductor device. Such semiconductor memory devices include those in which data can be electrically written and erased. So-called EEPROM and RAM exemplify a non-volatile memory device and a volatile memory device, respectively.
An EEPROM can retain data for a long time even when powered off, but has drawbacks that the number of rewriting data is restricted within a prefixed value, and that it requires several msecs. to conduct one data rewriting operation. Hence, an EEPROM is not suitable for the use in which data are rewritten frequently.
In a RAM including a DRAM, each data rewriting operation requires a short period of time (e.g., about 100 nsecs.), and the number of rewriting data is not restricted. But, a RAM cannot retain data when powered off.
To overcome these drawbacks of a conventional memory device, it has been proposed to combine an EEPROM with a DRAM by Y. TERADA et al (IEEE J. OF SOLID-STATE CIRCUITS, VOL. 23, NO. 1, FEB. 1988). In the proposed memory device, data can be transferred in a memory cell between an EEPROM portion and a DRAM portion. However, the proposed memory device has a relatively large size because the DRAM portion in each memory cell consists of two transistors and one capacitor.
In the proposed memory device, moreover, both terminals of the capacitor are connected to the control gate of a transistor of the EEPROM, resulting in that only the data stored in memory cells which are connected to the same word line can be transferred at a time, requiring a prolonged time for the data transfer.
In such a memory device wherein an EEPROM and a DRAM are combined, the EEPROM is directly connected with the DRAM. Such a configuration has a possibility that data once stored in the EEPROM may be destroyed when the DRAM is operated.
Recently, semiconductor optical sensors such as a CCD are widely used for converting optical signals into electric signals. Electric signals generated in such an optical sensor must be immediately conveyed to a suitable process means. In other words, a conventional optical sensor cannot store data produced therein.